Many chemical and physical vapor deposition methods are available to form coatings on substrates. For example, metals can be deposited by conventional evaporation or e-beam evaporation of a metal. Metal compounds (e.g., oxides, nitrides, and carbides) can be deposited by reactive chemical vapor deposition or reactive physical vapor deposition (e.g., sputtering). However, for chemical vapor deposition, it is particularly difficult to form certain metal oxides (e.g., yttrium oxide) because of the low vapor pressure of the starting materials (e.g., yttrium chloride). For reactive sputtering, it is particularly difficult to form certain metal oxides because of the formation of the oxide on the target metal.
Cathodic are plasma deposition methods, also referred to as cathodic are coating methods, produce generally uniform and dense coatings that are very hard and resistant to high temperatures. In cathodic are coating, an are, which is a discharge in a gas or vapor, is formed that is sustained by a highly ionized metal plasma produced by the erosion of the negative electrode (i.e., the cathode). Typically, the arc is active for a short period of time, momentarily extinguishes, and then reforms in a new location. This gives the appearance of the arc moving rapidly over a cathode surface. The resultant ebullition of the cathode material forms electrons, ions, macroparticles, and neutral vapor species. In reactive cathodic arc coating, a reactive gas (i.e., a process gas such as oxygen or nitrogen), is admitted into the coating chamber and an oxide or nitride is then deposited on the substrate. Typically, coatings formed from a cathodic arc method are generally dense reflective films having a small crystallite size (generally about 50-150 nm).
One problem encountered in cathodic arc coating methods results from the production of macroparticles in the plasma stream. Macropoarticles are large droplets (approximately 0.1-10 .mu.m) and solid fragments of the cathode material. These macroparticles produce undesirable flaws in the coating, particularly when coated on small diameter fibers. To prevent this, a combination of perpendicular magnetic and electric fields can be used to control the path of the plasma stream. The charged particles are directed toward the material to be coated and the macroparticles are allowed to continue on a line-of-sight path.
One means by which this is done is described in Kljuchko et al. U.S. Pat. No. 4,492,845. This patent describes a plasma arc apparatus comprising a working anode chamber in the form of a solid pipe of a nonmagnetic metal, and a consumable cathode provided with an opening for an object to be coated to be installed therein, wherein the cathode has a cross-sectional configuration that coincides with that of the anode pipe. The cathode is disposed within the anode in an axial relation so that the lateral surface of the cathode faces the anode and functions as an evaporation surface. A magnetic system for deviating ions of the plasma produced by the arc in the direction of the object to be coated is mounted on the outer side of the anode. In sum, the cathode surface, from which the vapor and macroparticles are formed, is placed out of the line-of-sight of the object to be coated. The magnetic field and its concomitant electric field turn a substantial fraction of the vapor toward the object to be coated, and the macroparticles fly off in a harmless direction. However, the fraction of the vapor that is not turned toward the object lands on the anode. This is undesirable because some of the macroparticles can be deflected back toward the object being coated. If an electrically insulating material (e.g., a metal oxide) is coated on the object, it will also coat the anode, thereby shielding the anode from the electrical current required to sustain the process. The system then shuts down. Typically, this occurs after less than about 10 minutes of operation. Because an important class of coatings are electrically insulating, this apparatus cannot be efficiently used for applying such coatings. Thus, a need exists for an apparatus capable of coating electrically insulating materials, as well as other materials, using cathodic arc methods.